Most pastelists have wet pastel in the initial stages of their paintings as a means of setting a tone upon which to work. This is especially useful when you work on a light paper and need to cover large areas with a darker value or when a major color shift needs to occur. Without this stain to set the general tone and value, a considerable amount of pastel will have to be applied to cover the undertone of the surface. This can fill the tooth of the surface and eventually make the painting appear heavy and muddy.
Depending on the surface, various liquids can be used. The three most popular are: water (purified or filtered is recommended due to the adverse consequences of chlorine and minerals often found in tap water); alcohol (hardware store denatured alcohol or drugstore rubbing alcohol); and OMS (odorless mineral spirits). It is advisable to test the liquid on a scrap of surface in advance of committing to a substantial painting. Surfaces that are high in rag content or fairly thin can be adversely affected by water. Pastel grit surfaces that rely on an acrylic binder may become softened. Some papers that have been mounted to add rigidity may have an adhesive that will release when exposed to certain liquids. Each of these liquids will interact with the pastel pigment in different ways, creating various characteristics. There are four components interacting to create the effect: the chosen pastel pigment, the surface, the liquid, and you.
Many pastelists comment that they have a hard time creating a vibrant underpainting with pastel. It often looks rich and strong when wet and then appears dull and weak when dry. One way of circumventing this is to use a mixed media for the undertone. Watercolor, acrylic, gouache, and liquid pigment are popular choices. It is of note that all pigments will appear richer and brighter when wet. With a little practice, you will become accustom to the dry-down and learn to compensate. If you wish to stay true to pastel, I recommend using the purest pigments within a color family. Avoid pastel sticks that have white or black added to tint and shade their appearance. These will often cloud other pigments within their proximity when wetted and dry to a cloudy appearance. Another consideration is the undertone of the surface and the relative thickness of the pastel application. The lighter the surface in value, the brighter the thinned pastel will appear. Think of the thinned pastel pigment as if it were a transparent medium like watercolor or thin oil glazes. In essence, when water or OMS is added to the pigment, that is what is created. Light needs to travel through the thinned layer of pigment and reflect back of the surface to produce brightness. To better control the volume of pastel application, harder pastel stick brands are often employed. These will deposit a minimum amount of pigment and help to avoid a thick opaque undertone that may fill the tooth of the surface, making subsequent pastel application difficult. While there are many fine hard pastel brands on the market, there is one that stands out for tinting strength and vibrancy: Faber-Castell Polychromos. They are extremely light-fast and available in 120 colors. If a vibrant pastel underpainting is desired, these are a must.
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